This invention relates to the testing of semiconductor rectifiers and, more particularly, to the testing of such rectifiers which are mounted on the rotors of brushless alternating current generators.
Alternating current synchronous generators of the brushless type are supplied with field excitation from an alternating current exciter and a rotating rectifier assembly. In such generator systems, the main generator has armature windings on the stator and the field winding on the rotor. A main alternating current exciter is provided which has its field winding on the stator and its armature winding on a rotor which is carried on the same shaft as the rotor of the main generator, or is otherwise mounted for rotation with that rotor. A rotating rectifier assembly is mounted on the same shaft, for rotation with the generator and exciter rotors, and is electrically connected between the exciter armature winding and the generator field winding. The stationary exciter field winding is supplied with direct current excitation from any suitable source, which may be independent of the main generator, such as a pilot exciter. In this way, a generator system is provided which requires no commutator or slip rings, and no brushes or sliding contacts.
In a typical brushless alternator for use in aerospace applications, the rotor turns at several thousand revolutions per minute. DC current in the stationary exciter field winding produces a three phase AC output on the rotor that is rectified by diodes mounted on the rotating shaft. The rectified output is supplied to the rotating field winding which is used to produce a rotating field that is used to generate the main AC output of the machine. The environment on the rotor is extremely hostile to the rotating rectifiers, combining high temperatures with high vibration and very high acceleration forces due to rotation. Consequently, failures of the rectifiers can occur. The rotating rectifiers usually fail shorted. This causes the alternator excitation requirements to increase by about 100%. The increased current causes overheating of the exciter and ultimately complete failure of the windings.
Circuits have been developed to detect diode failures on operating alternators, as illustrated by U.S. Pat. Nos. 3,210,603 and 3,534,228. Those circuits detect the ripple current induced in the exciter field winding by the unbalanced conduction of the rectifier bridge. The induced ripple current is at the fundamental exciter frequency and has a magnitude proportional to the load on the machine. To test alternators with those circuits, the alternators must be mechanically connected to an external drive source, such as an aircraft engine or test stand. This makes such testing time consuming. It is therefore desirable to develop a circuit for testing these rectifiers statically, that is, without rotating the alternator rotor.